1. Field of the Invention
The present invention relates to a finder of an optical device, such as a camera, and more particularly, to a real-image finder.
2 . Description of the Related Art
In recent years, size reduction of optical devices, such as cameras, has further advanced, and a principal object in this development is to reduce the space in the optical device occupied by the finder.
On the other hand, an increasing number of optical devices require switching of visual field masks of different sizes, and zooming of objective lens systems. In order to satisfy these requirements in a compact system, a real-image finder is most preferable.
The real-image finder requires an image-inverting optical system to allow viewing of an erect image. For the purpose of size reduction, an increasing number of image-inverting optical systems have a roof reflecting surface.
FIG. 7 is a schematic structural view of a finder using a roof prism proposed in Japanese Patent Application Laid-Open No. 9-105863.
Referring to FIG. 7, the finder includes an objective lens 10 for forming an object image on a predetermined plane, an image-inverting optical system P composed of a triangular prism P1 and a roof prism P2, and an eyepiece Le having positive retracting power.
A light beam from the objective lens 10 enters an incident surface P1a of the triangular prism P1, is reflected by a surface P1b toward a surface P1c that is flush with the incident surface P1a, and is totally reflected by the surface P1c to emerge from an emergent surface P1d. The light beam from the emergent surface P1d enters an incident surface P2a of the roof prism P2 via an indication member 5 having a visual field frame and the like, is totally reflected by a surface P2b to enter a roof surface P2c, is reflected back in the direction perpendicular to the observation field, is totally reflected by a surface P2d that is flush with the incident surface P2a to emerge from an emergent surface P2e that is flush with the surface P2b, and is guided to an eyepoint E via the eyepiece Le.
In this way, an inverted object image formed by the objective lens 10 is inverted vertically and horizontally by the image-inverting optical system P, and is observed as an erect image from the eyepoint E via the eyepiece Le.